Fuel elements for nuclear reactors are used in, or are being considered for many applications such as by the commercial nuclear power industry, by the military for nuclear powered submarines and ships, and by the aerospace industry for outer space systems. Specifically, the fuel elements used in hot hydrogen gas propelled nuclear thermal propulsion (NTP) engines may contain a graphite (Gr) Gr based substrate. In these applications, cryogenic hydrogen is heated to a high temperature by a nuclear reactor. The pre-heated hydrogen working fluid may enter the NTP engine at about 355-370 K, and may be further heated to 2500-2700 K in the nuclear reactor, where it expands through a rocket nozzle to create thrust. Thus, the energy from the nuclear reactor is used in lieu of the chemical energy produced by reactive chemicals in a chemical rocket. Nuclear thermal rockets can have a higher effective exhaust velocity and higher propulsion efficiency than conventional chemical rockets.
However, uncoated UC (uranium carbide) nuclear fuel embedded in either a Gr or a Gr/ZrC (graphite/zirconium carbide) substrate, is susceptible to hot hydrogen attack of the Gr-based substrate. This hot hydrogen corrosive (erosive) attack occurs when the hydrogen reacts with the Gr-based substrate to form methane and other hydrocarbons. As a result, the Gr-based substrate erodes with time, which affects the reactor neutronics and performance leading to a premature shut-down of the NTP engine.
Under the project Rover and Nuclear Engine Rocket Vehicle Application (NERVA) programs, a protective NbC (niobium carbide) or ZrC coating was deposited in the inner cooling channels of the fuel elements through which the hydrogen propellant flows. The coating was applied by chemical vapor deposition (CVD). During the later stages of these programs, the ZrC coating was shown to be a more effective protective coating than the NbC coating, so that ZrC was the preferred coating.
FIG. 1 shows the schematic layout of an array of coated coolant channels in the fuel elements. Although these coatings proved to be effective in improving engine performance and increasing its operating life over using uncoated fuel elements, these coatings were not completely effective in protecting the Gr-based substrates since the coefficients of thermal expansion (CTE) of NbC and ZrC are higher than those of Gr as shown in Table 1 below.
TABLE 1Coefficients of thermal expansion for Gr, NbC, uranium carbide and ZrCMaterialT (K)α (× 10−6) (K−1)Gr (GrafTech XTE 70)*373-22734.7-6.9 NbC293-30005.7-11.4UC293-20009.8-12.6ZrC293-30004.0-10.2* It is noted that the CTE data varies depending on how it was manufactured.
It is clear that adding UC to the Gr would lead to larger differences in the CTE mismatch between the ZrC coating and the substrate. As a result, significant mass loss occurred along the length of the fuel rod closer to the hydrogen gas inlet end, which has been termed “mid-passage corrosion (erosion)”. During the latter stages of the Rover/NERVA programs, (U,Zr)C composite fuels were developed to minimize the CTE mismatch between the ZrC coating and the substrate, and while this solution helped to some extent, it did not prevent the occurrence of mid-passage corrosion. Therefore, there is a need to address the mismatch between the CTE of the ZrC coating and that of the (U,Zr)Gr composite fuel substrates in NTP and other applications. Further there is a need for improved structures, compositions of matter and methods for protecting Gr-based substrates of nuclear fuel elements.